1. Field of the Invention
The invention generally relates to a technique of Code Division Multiple Access (CDMA), and more particularly, to a base-station antenna apparatus and a method of optimizing traffic capacity of the cell in a CDMA communications system.
2. Description of the Related Art
In the CDMA communication system, multiple channels independently occupy the same frequency band, and all signals are transmitted simultaneously in time. The different channels (or waveforms) in CDMA are distinguished from one another at the receiver using the specific spreading codes they employ. In CDMA, all the signals other than the target channel become interference signals. Since an excessive amount of interference prevents good communication conditions, the number of permissible users (i.e., the traffic capacity) in the cell or the sector is inevitably limited. To this end, an appropriate measure for reducing interference is required in order to increase the traffic capacity as much as possible.
One known technique for reducing interference from other communication channels is cell sectoring. With this technique, the cell under the control of the base station is divided into multiple sectors, and the base station is furnished with multiple directional antennas with different beam patterns, each antenna corresponding to one of the sectors. The main beam of each directional antenna is turned to the associated sector, while the side lobe is turned to other sectors, in order to spatially separate the expected signal (from the target channel) from the interference signals (from other channels).
FIG. 1 is a perspective view of a directional antenna 100 used in the cell sectoring technique, and FIG. 2 shows its plan view. The directional antenna 100 has a rod-type radiator 103, a pair of reflector plates 102 arranged with an angle a between them in order to reflect the radio waves emitted from the radiator 103, and the rotating unit 101. The rotating unit 101 rotates the assembly of the radiator 103 and the reflector plates 102 about an axis parallel to the radiator rod, while maintaining the positional relationship between the radiator 103 and the reflector plates 102, to change the direction β of the beam orientation. A reflector angle adjustor (or a reflector-plate opening/closing unit) 105 is provided behind the reflector plates 102. The reflector angle adjustor 105 changes the angle α between the two plates 102 to change the beam width (or the beam pattern) of the directional antenna 100.
FIG. 3A illustrates an antenna assembly using three directional antennas 100A, 100B, and 100C, which is used for a three-sector cell. The beam pattern of each directional antenna is directed to the associated sector, as illustrated in FIG. 3B.
FIG. 4A illustrates an antenna assembly using six directional antennas 100A through 100 F, which is used for a six-sector cell. The beam pattern of each directional antenna is directed to the associated sector, as illustrated in FIG. 4B.
The cell sectoring technique using these antenna assemblies is advantageous in the multi-path transmission environment with large transmission delay or spread in the direction of signal arrival, to reduce the interference from the other communication channels. However, in the multi-path transmission environment with little transmission delay or spread in the direction of arrival, the cell sectoring technique is not so advantageous. Especially, in the environment in which various radio signals having different communication qualities or transmission types (represented by the transmission power, the transmission rate, the error rate, etc.) exist, the cell sectoring techniques is inferior to the adaptive antenna array technique in its interference reducing ability.
The adaptive antenna array technique is another technique for reducing the interference from other channels. With this technique, multiple antenna elements are arranged in a predetermined configuration to form an antenna array in the base station. These antenna elements are controlled adaptively using known signals, such as a pilot bit or a unique word contained in the received signal, so as to maximize the signal to interference ratio (SIR). The interference can be spatially separated from the target signal.
FIG. 5 illustrates an example of the adaptive antenna array, and FIG. 6 illustrates the directivity of this adaptive antenna array. The adaptive antenna array 500 illustrated in FIG. 5 has n antenna elements (5011, 5012, . . . , 501n), and weighting means that multiply the signal received at each of the antenna elements by one of the associated weighting coefficients (w1, w2, . . . , wn). The weighted signals are synthesized and a synthetic signal is output from the output terminal. Each of the weighting coefficients can be adjusted and varied so that the SIR of the synthetic signal becomes the maximum. This arrangement can achieve the optimum directivity.
In FIG. 6A, the directivity of the adaptive antenna array using four antenna elements (n=4) is illustrated. In this example, the antenna elements are rod-type radiator elements, which are aligned in a line to form an antenna array, as illustrated in FIG. 6B. In general, (n−1) null points are produced at the front face of the adaptive antenna array. In this example, three (4−1=3) null points are formed at the front face of the antenna array, as illustrated in FIG. 6A. These null points are directed to channels other than the target channel to achieve large interference reducing effect.
The adaptive antenna array technique is advantageous in reducing the interference, while increasing the traffic capacity, especially in the environment in which different qualities or different types of signals coexist at different transmission powers, different transmission rates, or different error rates. This is because the directivity characteristics of the antenna array, including the main lobe and the position of the null points, are appropriately changed so as to maximize the SIR. For example, by guiding a signal with large transmission power onto the null point, the communication quality of a low-transmission-power signal can be guaranteed.
However, in another multi-path transmission environment with wide spread of delay time or arrival directions in which transmission qualities or transmission types are similar to each other at similar transmission power levels, transmission rates, and error rates, the adaptive antenna array technique is inferior to the cell sectoring technique in ability to reduce interference. This is because, if there is a large spread of transmission delay or signal arrival direction, then the arrival direction of the target channel is dispersed. To respond to the dispersion of the arrival direction, the main lobe has to be formed wide. This causes the spatial separation of the target signal from the interference to be reduced. In addition, the interference reducing effect making use of the null points cannot be expected because the signal arrival directions of the other channels are also dispersed. Another reason for the reduction of the interference reducing effect is that the communication qualities or the transmission types of the signals are similar to each other, and therefore, it is difficult to find and bring a signal with large transmission power to a null point.
Using the above-described known techniques independently cannot respond to the change in the communication environment (including traffic conditions and communication qualities) in a flexible manner, and accordingly, it cannot optimize the traffic capacity in the given frequency band.